Abstract
(Ti0.5, Mg0.5)N thin films were synthesized by reactive dc magnetron sputtering from elemental targets onto c-cut sapphire substrates. Characterization by θ–2θ X-ray diffraction and pole figure measurements shows a rock-salt cubic structure with (111)-oriented growth and a twin-domain structure. The films exhibit an electrical resistivity of 150 mΩ·cm, as measured by four-point-probe, and a Seebeck coefficient of −25 µV/K. It is shown that high temperature (~800 °C) annealing in a nitrogen atmosphere leads to the formation of a cubic LiTiO2-type superstructure as seen by high-resolution scanning transmission electron microscopy. The corresponding phase formation is possibly influenced by oxygen contamination present in the as-deposited films resulting in a cubic superstructure. Density functional theory calculations utilizing the generalized gradient approximation (GGA) functionals show that the LiTiO2-type TiMgN2 structure has a 0.07 eV direct bandgap.
Highlights
Titanium nitride-based hard coatings [1,2,3] have a long history for use in applications such as protective layers and hard coatings
Among the many TiN-based compounds, titanium-magnesium nitride (Ti0.5, Mg0.5)N [9,10,11] is of particular interest due to its semiconducting properties, which motivates research on electronic and energy-related applications
The relatively high thermal conductivity [23,24,25,26] of approximately 8–12 Wm−1·K−1 results in a low thermoelectric figure of merit. To address this issue, Alling [27] studied (Ti1−x, Mgx)N alloys and a hypothetical semiconducting TiMgN2 superstructure using density functional theory (DFT) and proposed that by replacing the group-3 element scandium with a group-2 alkaline earth element and a group-4 transition metal, it is possible that the resulting compound will have a similar power factor to that of scandium nitride (ScN), but with a reduced thermal conductivity
Summary
Titanium nitride-based hard coatings [1,2,3] have a long history for use in applications such as protective layers and hard coatings. The relatively high thermal conductivity [23,24,25,26] of approximately 8–12 Wm−1·K−1 results in a low thermoelectric figure of merit (zT) To address this issue, Alling [27] studied (Ti1−x, Mgx)N alloys and a hypothetical semiconducting TiMgN2 superstructure using density functional theory (DFT) and proposed that by replacing the group-3 element scandium with a group-2 alkaline earth element (magnesium) and a group-4 transition metal (titanium), it is possible that the resulting compound will have a similar power factor to that of ScN, but with a reduced thermal conductivity. In a previous paper [31], we studied different crystal structures with an ABN2 stoichiometry (A and B being elements), concluding that stoichiometric and elementally pure TiMgN2 is stable in the NaCrS2 superstructure (which is equivalent to the B1-L11 depending on viewpoint direction) and predicted its thermoelectric power factor to be larger compared to ScN. In addition to studying (Ti0.5, Mg0.5)N and its thermoelectric properties, post-annealed (Ti0.5, Mg0.5)N are investigated for the formation of any superstructure and/or other secondary phases
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